For the production of highly qualified dissolving pulp there is usually used pre-hydrolized, bleached sulphate pulp. Different bleaching procedures give their specific advantages and draw-backs. If only a certain number of the bleaching steps, including chlorination, alkali extraction, chloride dioxide bleaching, or hypochlorite bleaching is used there is obtained a standard cord pulp, the R18-value of which has an average who will reach about 96-97%. If the purpose is to produce a cord pulp with higher quality it is now conventional in the bleaching sequence to apply a so called cold alkalization step, which means that the usually finish-bleached pulp is treated with NaOH at a concentration of 80-110 g/l at a temperature of 20.degree.-30.degree. C. During this treatment the cellulose fibres swell heavily and a great portion hemicellulose is dissolved. R18-values of 97-99% may be reached, which is very high.
When the pulp is neutralized and washed after the cold alkali refinement the fibre will shrink again. Two new features now however are introduced into the fibre:
The fibre is no longer straight but bent and curled; the fibre wall also has become more porous. Therefore the fibre will swell and increase its dimension much faster and much more when it again is brought into contact with highly concentrated alkaline solution than a not cold refined fibre.
In the case of viscose cellulose production the pulp after drying is cut into sheets with dimensions fitting in the mercerizing presses of the viscose factory. The pulp sheets are arranged vertically in the mercerizing presses whereupon 18% NaOH solution is pumped in from underbeneath. The sheets swell heavily at the same time as they are increasingly weakened. One aims at pumping in the sodium hydroxide solution at such a rate that the surface of the liquor permanently will lie 2-3 mm below the front of the liquor which is sucked up into the sheets. To the extent the sheets swell more the supply of liquor to the front from underbeneath via the space between the sheets is increasingly shut off. The supplied liquor therefore can pass into the central parts of the package of the sheets only in the "ditch" which is formed between the sheets as long as the sheets are thin in their dry or still unswelled portion above and just below the surface of the liquor, respectively, and thick in their swelled portion at greater depths. If the front will exceed upwards faster than the rate with which the liquor may flow to the narrow ditches there will be obtained a lower liquor level in the central portion of the sheets than outside the package. In other words one will obtain an increasing dynamic capillary rise. When the solution get poorer and poorer in NaOH, which will occur when the dynamic capillary rise is great, and the NaOH concentration is reduced to below 180 g NaOH/l, the swelling of the sheets will increase heavily as cellulose has its maximum swelling capacity at about 100 g NaOH/l. Therefore the spaces between the sheets will be more and more blocked for the liquor and eventually one can reach a catastrophic situation.
Because of the cold refining treatment of the cellulosic pulp the pulp will obtain considerable elasticity. The pulp mills therefore have been forced to install large, energy consuming refiners for beating the pulp prior to the pulp dryer. The pulp mills also have been forced to install very large and expensive smothing presses in the pulp drying machines as well as large baling presses. In spite of these large investment costs one has however still to meet great technical problems. If the pulp is beaten too much prior to the pulp dryer there is formed crill, which will cause impaired de-watering capacity and low production on the pulp dryer, as well as bad pressability and hence low production in the viscose factories.